Equals and GetHashCode The Right Way!!!!!

Hi
Recently I witness many mistakes when creating new class. many developers override Object.GetHashCode() in a wrong way!!! that can make error that will be very hard to find. Lets start with the basic rules when writing your own GetHashCode:

• If X.Eqauls(Y) == true then X.GetHashCode() == Y.GetHashCode(). if two objects are equals they must have the same HashCode.
• If X.GetHashCode() != Y.GetHashCode() then X.Equals(Y) == false. If two objects has defraent hash code they cant be Equal to each other.
• object GetHashCode() must NOT be changed during it life cycle. Once the object was created it hash code must NOT change!!!
• If X.Eqauls(Y) == false then X.GetHashCode() Should (NOT A MUST) be defrant from Y.GetHashCode()

If you creates your own object and overrides Equals you will get an Warning:
Here is an example of a wrong way to make HashCode and what will happen:

class MyWrongObject

    {
    private int m_One;
    public int One
    {
        get { return m_One; }
        set { m_One = value; }
    }
    public MyWrongObject() { }
    public MyWrongObject(int one)
    {
        m_One = one;
    }

    public override bool Equals(object obj)
    {
        MyWrongObject other = obj as MyWrongObject;
        if (other != null)
            return (One == other.One);
        else
            return false;
    }

    public override string ToString()
    {
        return String.Format("MyWrongObject[{0}]", One);
    }

    public override int GetHashCode()
    {
        return ToString().GetHashCode();
    }


    public static void Main(string[] args)
    {
        HashSet<MyWrongObject> set1 = new HashSet<MyWrongObject>();
        MyWrongObject obj1 = new MyWrongObject(1, 1);
        set1.Add(obj1);
        if (set1.Contains(obj1))
            Console.WriteLine("Found obj1");
        obj1.One = 2; // This line changes the GetHashCode() of obj1
        if (set1.Contains(obj1))
        {
            // We wont see this line because Contains looks at the GetHashCode()
                then at Equals()
            Console.WriteLine("You won't see this line");
        }
        else
            Console.WriteLine("Hmmm.. can't find obj1 but obj1 is here");
        foreach (MyWrongObject obj in set1)
        {
            if (obj1.Equals(obj))
                Console.WriteLine("obj:{0} Can't find obj1 but obj1 is here", obj);
        }
    }
}

Please notice that you can't make a setter to a member that changes the GetHashCode() of the object.
If you do that you should think about throwing an exeption when using GetHashCode()

Enjoy,